1. The D325J output can only be connected to passive speakers, not active speakers or constant-voltage loudspeakers. 2. The D325J uses a DC 24V-48V power supply. Higher voltage results in higher power output, but the voltage must not exceed 48V, otherwise the amplifier be damaged. The power connector is 5.5*2.5, with positive polarity on the inside and negative on the outside. 3.The device has a startup delay of approximately for preheating the vacuum tubes; sound be produced after the tubes light up. To turn off the device, turn the volume knob to the minimum position; the indicator ring turn off. 4. The negative terminals of the left and right channels of the amplifier output must not be connected together, otherwise the amplifier be damaged. 5. The USB input can be connected to a mobile phone's USB port or a computer; simply connect it to play audio, no driver installation is required. When connecting a mobile phone via USB, an OTG cable is required; a charging cable cannot be used. USB flash drives cannot be connected. Due to different batches, there may be differences in product appearance, color, and printed text, which not affect the use. Color: Black Material: Aluminum Alloy Package Contents: 1*Digital Amplifier Only the above package content, other products are not included. Note: Light reflection and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.